Surface-emitting laser

ABSTRACT

Provided is a high-output surface-emitting laser capable of reducing effects on reflectance of an upper reflection mirror in a single transverse mode. The surface-emitting laser includes plural semiconductor layers, laminated on a substrate, which includes a lower semiconductor multilayer reflection mirror, an active layer, and an upper semiconductor multilayer reflection mirror, wherein the lower or upper semiconductor multilayer reflection mirror includes a first semiconductor layer having a two-dimensional photonic crystal structure comprised of a high and low refractive index portions which are arranged in a direction parallel to the substrate, and wherein a second semiconductor layer laminated on the first semiconductor layer includes a microhole which reaches the low refractive index portion, the cross section of the microhole in the direction parallel to the substrate being smaller than the cross section of the low refractive index portion formed in the first semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a division of U.S. application Ser. No.12/166,225, filed Jul. 1, 2008, the entire disclosure of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface-emitting laser.

2. Description of the Related Art

A vertical cavity surface-emitting laser (hereinafter, referred to asVCSEL) is a laser for emitting a laser beam in a direction perpendicularto an in-plane direction of a semiconductor substrate.

A distributed Bragg reflector (hereinafter, referred to as DBR) isnormally used as a reflective layer of the surface-emitting laser.

The DBR is generally formed by alternately laminating a high refractiveindex layer and a low refractive index layer with an optical filmthickness of λ/4.

The surface-emitting laser has such excellent characteristics that astable single mode is obtained as a longitudinal mode characteristic, athreshold value thereof is lower than a threshold value of anedge-emitting laser, and a two-dimensional array is easily formed.

Therefore, it is expected that the surface-emitting laser will beapplied as a light source for optical communication and opticaltransmission or a light source for electrophotography.

In order to enhance applicability of the VCSEL, a VCSEL which produces ahigher output while maintaining a single transverse mode oscillation isdesired.

Accordingly, various structures have been considered, and as one of thepromising structures, Song et al., Applied Physics Letters Vol. 80, p.3901 (2002) (hereinafter, referred to as Document 1) proposes a photoniccrystal VCSEL in which a two-dimensional photonic crystal structure of aphotonic crystal fiber structure is formed in VCSEL.

FIG. 5 illustrates a structure of a surface-emitting laser described inDocument 1.

In a surface-emitting laser 600 illustrated in FIG. 5, a lowermultilayer reflection mirror 610, a lower spacer layer 620, an activelayer 630, an upper spacer layer 640, and an upper multilayer reflectionmirror 650 are laminated on a substrate 605.

When voltage is applied to an upper electrode 690 formed on the uppermultilayer reflection mirror 650 and to a lower electrode 695 formedunder the substrate 605, the active layer 630 emits light, and theemitted light is amplified by a resonator formed of the upper reflectionmirror and the lower reflection mirror, whereby a laser oscillation isobtained. As a result, a laser beam is emitted in a directionperpendicular to the substrate.

In a part of the upper multilayer reflection mirror 650, there is formeda current confinement structure 660 including a conductive region 661and a high resistance region 662.

The current confinement structure is formed through oxidation of anAlGaAs layer or an AlAs layer which has a high Al compositional ratio.

AlxOy which is formed through oxidation of AlGaAs or AlAs has a higherelectrical resistance and a lower refractive index compared with AlGaAsor AlAs.

From an upper surface of the upper multilayer reflection mirror 650 tothe active layer side, a two-dimensional photonic crystal structureincluding multiple holes 675 is formed. A defect is provided in a centerof the two-dimensional photonic crystal structure.

In a region where the two-dimensional photonic crystal structure isformed, the effective refractive index decreases.

Here, the amount of decrease in effective refractive index describedabove is less than the amount of decrease in effective refractive indexobtained in the region where AlGaAs or AlAs is oxidized.

In optical confinement caused by a refractive index difference, thesmaller a refractive index difference is, the larger an area of a waveguide portion where a single transverse mode can be maintained is.

Accordingly, current confinement is conducted by an oxidized apertureand optical confinement in a horizontal direction is conducted by atwo-dimensional photonic crystal structure, whereby an emitting area canbe increased while maintaining a single transverse mode oscillation,compared with the case where both of the confinement are conducted bythe oxidized aperture. In the aforementioned surface-emitting laser ofDocument 1, the defect size of the two-dimensional photonic crystalstructure is made smaller than the current confinement size, with theresult that a surface-emitting laser which maintains the singletransverse mode and has a larger emitting area can be realized.

However, in the structure where a hole of the two-dimensional photoniccrystal is formed from the surface of the upper multilayer reflectionmirror, as in the case of Document 1, a deep hole needs to be made forachieving sufficient transverse mode control.

This is because the resonating region having a large light intensity ispositioned on the active layer side of the upper multilayer reflectionmirror, so the transverse mode control cannot be exhibited sufficientlywithout the two-dimensional photonic crystal structure.

However, when a deep hole is prepared, the refractive index changes overa long distance in a perpendicular direction within the upper multilayerreflection mirror, which leads to an increase in shift amount of aresonance wavelength of the reflection mirror.

As a result, the reflectance of the upper reflection mirror decreasesfor a resonance laser beam, which increases reflection loss.

For this reason, a greater emitting area can be secured but resonatorperformance decreases in the transverse mode control structure, and thusoutput cannot be sufficiently increased.

SUMMARY OF THE INVENTION

In view of the above-mentioned problem, an object of the presentinvention is to provide a high-output surface-emitting laser capable ofreducing effects on a reflectance of an upper reflection mirror in asingle transverse mode.

Therefore, in order to solve the above-mentioned problem, the presentinvention provides a surface-emitting laser having a structure describedbelow.

The surface-emitting laser according to the present invention includes aplurality of semiconductor layers laminated on a substrate, thesemiconductor layers including a lower semiconductor multilayerreflection mirror, an active layer, and an upper semiconductormultilayer reflection mirror, wherein one of the lower semiconductormultilayer reflection mirror and the upper semiconductor multilayerreflection mirror includes a first semiconductor layer having atwo-dimensional photonic crystal structure comprised of a highrefractive index portion and a low refractive index portion which arearranged in a direction parallel to the substrate, and wherein a secondsemiconductor layer laminated on the first semiconductor layer includesa microhole which reaches the low refractive index portion, the crosssection of the microhole in the direction parallel to the substratebeing smaller than the cross section of the low refractive index portionformed in the first semiconductor layer.

According to the present invention, the high-output surface-emittinglaser capable of reducing effects on a reflectance of the upperreflection mirror in a single transverse mode can be realized.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory view illustrating a surface-emittinglaser according to Embodiment 1 of the present invention.

FIG. 2 is a schematic explanatory view illustrating a surface-emittinglaser according to Embodiment 2 of the present invention.

FIG. 3 is a schematic explanatory view illustrating a surface-emittinglaser according to Embodiment 3 of the present invention.

FIG. 4 is a view illustrating formation of a resist pattern in aphotonic crystal structure of the surface-emitting laser according toEmbodiment 1 of the present invention.

FIG. 5 is a schematic explanatory view illustrating a surface-emittinglaser according to a conventional case.

DESCRIPTION OF THE EMBODIMENTS

Next, embodiments of the present invention are described.

The following description is given of the embodiments of asurface-emitting laser according to the present invention, in whichmultiple semiconductor layers including a lower semiconductor multilayerreflection mirror, an active layer, and an upper semiconductormultilayer reflection mirror are laminated on a substrate, withreference to the drawings.

Note that, in FIG. 1 to FIG. 4 described below, the same orcorresponding parts are denoted by the same reference numerals.

Embodiment 1

A surface-emitting laser according to Embodiment 1 of the presentinvention is described.

FIG. 1 is a schematic explanatory view illustrating the surface-emittinglaser according to this embodiment.

In FIG. 1, a surface-emitting laser 100 includes a substrate 105, aresonator 107, a lower semiconductor multilayer reflection mirror 110, alower spacer layer 120, an active layer 130, an upper spacer layer 140,an upper semiconductor multilayer reflection mirror 150, a currentconfinement layer 160, a two-dimensional photonic crystal structure 170,and a low refractive index portion 175.

The surface-emitting laser 100 includes a microhole 180, an upperelectrode 190, and a lower electrode 195.

In this embodiment, the surface-emitting laser 100 has a structure inwhich the resonator 107 is formed on the substrate 105.

In the resonator 107, the lower semiconductor multilayer reflectionmirror 110, the lower spacer layer 120, the active layer 130, the upperspacer layer 140, and the upper semiconductor multilayer reflectionmirror 150 are formed in the stated order.

The upper semiconductor multilayer reflection mirror 150 includes thecurrent confinement layer 160 formed of a conductive region 161 and ahigh resistance region 162.

Further, a first semiconductor layer forming the upper semiconductormultilayer reflection mirror 150 is provided with the low refractiveindex portion 175 having a lower refractive index compared with thesurrounding semiconductor layers, whereby a two-dimensional photoniccrystal structure having a two-dimensional refractive index distributionin a direction parallel to the substrate is formed.

In other words, through formation of the low refractive index portion175, there is formed the two-dimensional photonic crystal structure 170formed of a high refractive index portion and a low refractive indexportion which are arranged in the direction parallel to the substrate.

A second semiconductor layer is laminated on the first semiconductorlayer, and the microhole 180 penetrates from an upper surface of the lowrefractive index portion 175 to an upper surface of the secondsemiconductor layer, that is, to a surface of the upper semiconductormultilayer reflection mirror 150.

The cross section of the microhole in a direction parallel to thesubstrate is made smaller than the cross section of the low refractiveindex portion of the first semiconductor layer in the direction parallelto the substrate. Further, the ring-shaped upper electrode 190 isprovided on the surface of the upper semiconductor multilayer reflectionmirror 150, and the lower electrode 195 is provided on the substrate105.

In the surface-emitting laser 100, when a voltage is applied between theupper electrode 190 and the lower electrode 195, the active layer 130emits light, and light leaking from the active layer 130 is resonatedand amplified by the resonator 107. Accordingly, a laser beam issurface-emitted from the upper surface of the upper semiconductormultilayer reflection mirror 150. As the two-dimensional photoniccrystal structure 170 of this embodiment, a photonic crystal fiberstructure is desirable. In the photonic crystal fiber structure, thelight propagation axis serves as a core, and multiple low refractiveindex portions which have a lower refractive index compared with theupper semiconductor multilayer reflection mirror form a cladding portionaround the core.

The current confinement structure can be manufactured in the same manneras a method employed in manufacturing a conventional vertical cavitysurface-emitting laser (VCSEL).

In other words, as a typical method, proton ion implantation orselective oxidation can be used.

According to the surface-emitting laser of the aforementionedembodiment, a transverse mode can be controlled by the two-dimensionalphotonic crystal structure.

Further, as described above, the cross section of the microhole in thedirection parallel to the substrate is made smaller than the crosssection of the low refractive index portion of the first semiconductorlayer in the direction parallel to the substrate. As a result, comparedwith the conventional structure in which a hole of the two-dimensionalphotonic crystal structure is directly made from the surface of theupper multilayer reflection mirror, reduction in resonance performancecan be suppressed.

That is, compared with the case where the hole of the two-dimensionalphotonic crystal structure is allowed to penetrate from the surface byemploying the aforementioned structure, a less volume of the hole isrequired. Thus, wavelength shift of the reflection mirror can bereduced.

Further, surface roughness of the formed hole may cause scattering lossof resonance light on an interface, but its surface area can also bereduced. Accordingly, a reduction in reflectance of the multilayerreflection mirror can be suppressed, whereby reduction in lifetime ofthe light in the surface-emitting laser resonator can be suppressed. Asa result, high-output operation can be realized.

Further, according to the surface-emitting laser of the aboveembodiment, the upper multilayer reflection mirror is a semiconductor,and thus the upper multilayer reflection mirror is provided withconductivity. Accordingly, the structure in which an electrode isprovided on the surface of the upper multilayer reflection mirror can berealized.

Next, a method of manufacturing the surface-emitting laser 100 accordingto this embodiment is described. In this embodiment, as the substrate105 of the surface-emitting laser illustrated in FIG. 1, for example, ann-type semiconductor substrate such as an n-GaAs substrate can be used.

On the n-type semiconductor substrate 105, the lower multilayerreflection mirror 110, the lower spacer layer 120, the active layer 130,the upper spacer layer 140, and the upper multilayer reflection mirror150 are subsequently laminated as described below.

First, multiple pairs of a low refractive index layer and a highrefractive index layer are laminated on the substrate 105 to form thelower multilayer reflection mirror 110.

The low refractive index layer and the high refractive index layer arelaminated using, for example, metal organic chemical vapor deposition(MOCVD).

The laminated structure of the low refractive index layer and the highrefractive index layer can be appropriately selected within the rangewhere a light having a laser oscillation wavelength is not absorbed.

In this case, the following laminated structure can be employed from theviewpoints of being transparent to the light having a wavelength of 670nm and securing a greater refractive index difference between the lowrefractive index layer and the high refractive index layer in order toobtain a high reflectance with a small number of pairs.

That is to say, an n-Al0.93Ga0.07As layer with a thickness of 49 nm isused as the low refractive index layer, and an n-Al0.5Ga0.5As layer witha thickness of 54 nm is used as the high refractive index layer, whereby70 pairs thereof are laminated.

Then, on the multilayer reflection mirror 110, the lower spacer layer120, the active layer 130, and the upper spacer layer 140 are laminatedby using, for example, MOCVD.

As the lower spacer layer 120, an n-type semiconductor, for example,n-Al0.93Ga0.07As is used.

As the active layer 130, for example, GaInP/AlGaInP having the quantumwell structure is used from a viewpoint of having optical gain at awavelength of 670 nm.

As the upper spacer layer 140, a p-type semiconductor, for example,p-Al0.93Ga0.07As is used.

The lower spacer 120, the active layer 130, and the upper spacer layer140 are laminated so that the total of their optical thicknesses issubstantially equal to, for example, a laser oscillation wavelength.

Next, on the upper spacer layer 140 for forming the current confinementstructure, 40 pairs of the low refractive index layer and the highrefractive index layer are laminated to form the multilayer reflectionmirror 150.

Now, the current confinement layer 160 for forming the currentconfinement structure in a part of the multilayer reflection mirrorstructure is formed.

As the current confinement layer 160 for forming the current confinementstructure, AlGaAs having a high Al compositional ratio, for example,p-Al0.98Ga0.02As is allowed to grow by 20 nm (the low refractive indexlayer formed in a second pair from the bottom is formed ofAl0.98Ga0.02As).

Further, on the upper side of the current confinement layer 160 withinthe multilayer reflection mirror structure, the first semiconductorlayer for forming the two-dimensional photonic crystal structure 170 islaminated. The thickness of the first semiconductor layer is, forexample, an optical thickness of 3.25 times an oscillation wavelength.

Then, on the first semiconductor layer, the second semiconductor layerfor forming multiple microholes 180 is laminated.

The low refractive index layer and the high refractive index layer canbe appropriately selected from the materials described above.

For instance, a p-Al0.93Ga0.07As layer with a thickness of 49 nm is usedas the low refractive index layer and a p-Al0.5Ga0.5As layer with athickness of 54 nm is used as the high refractive index layer tolaminate 40 pairs thereof.

In that case, the first semiconductor layer formed of the AlGaAs layeris made to have an Al compositional ratio which is higher than the Alcompositional ratio of the second semiconductor layer and higher thanthe Al compositional ratio of the semiconductor layer formed under thefirst semiconductor layer.

Next, in order to form the multiple microholes 180, a resist is appliedonto the surface of the second semiconductor layer to form atwo-dimensional photonic crystal pattern on the applied film.

For example, as illustrated in FIG. 4, a resist pattern, in which theresist is not removed for one circle in a place corresponding to theposition where an emitting portion 420 is provided, is formed in a basicmicrohole pattern 410. In the basic microhole pattern 410, a unit form,in which the resist is removed in a shape of a circle having a diameterof 250 nm at three vertices of an equilateral triangle having a side of2.5 μm, is periodically repeated.

Next, using ICP etching method which introduces chlorine gas, themultiple microholes 180 are formed in the second semiconductor layer (adiameter thereof is, for example, 50 nm p).

Here, a bottom of the microhole is made to position on the upper surfaceof the first semiconductor layer which forms the upper semiconductormultilayer reflection mirror 150 for forming the two-dimensionalphotonic crystal structure 170.

Next, the low refractive index portion 175 having a lower refractiveindex compared with the surrounding semiconductor layers is formed inthe first semiconductor layer by an oxidized region which is oxidized byoxidizing species supplied through the microhole.

For example, after the resist is removed, water vapor is introducedthrough the microhole, and then the first semiconductor layer issubjected to heat treatment at 450° C. As a result, in the firstsemiconductor layer, the two-dimensional photonic crystal structurehaving the low refractive index portion 175 (for example, low refractiveindex portion having a cross section corresponding to 200 nm p) providedwith a cross section, which is larger than the cross section of themicrohole 180, in a direction parallel to the substrate, is formed asdescribed below.

In other words, a portion which is in contact with the firstsemiconductor layer is oxidized, whereby an Al oxide having a lowerrefractive index compared with the surrounding semiconductor layers isformed. The Al oxide thus formed forms a region corresponding to the lowrefractive index portion 175 of the two-dimensional photonic crystal.

The size of the low refractive index portion can be adjusted by changingconditions of the oxidation process, that is, processing time, flow rateof water vapor, and temperature of heat treatment.

In this case, the size of a low refractive index portion can be set to acircular region having a diameter of, for example, 1 μm.

The microhole is filled with, for example, SiO₂ or a resin, and then acircular mesa having a diameter of 30 μm is formed.

The center of the circular mesa and the center of the two-dimensionalphotonic crystal pattern (in FIG. 4, center of a defect of the photoniccrystal) are made to coincide with each other.

A bottom of the mesa is allowed to reach the lower multilayer reflectionmirror.

Water vapor is introduced from an end surface of the current confinementlayer (high resistance layer) 160 made of p-Al0.98Ga0.02As, which isexposed to a mesa sidewall, and the current confinement layer 160 issubjected to heat treatment at 450° C., whereby an Al_(x)O_(y) the highresistance region 162 is formed.

In this case, the oxidation time is controlled so as to leave theconductive region 161 having a cross section of 100 μm² in the center ofthe mesa, thereby forming the current confinement structure.

Next, a polyimide protective film is formed in the vicinity of the mesa,and further the p-type upper electrode 190 is provided on an uppersurface of the mesa being connected therewith.

The upper electrode is made of, for example, Ti/Au. An electrodeprovided in a region within 10 μm of the diameter from the center of themesa is removed for light extraction by a lift-off method.

Finally, the n-type lower electrode 195 is formed on a rear side of thesubstrate 105, whereby the surface-emitting laser 100 oscillating at awavelength of 670 nm, where a transverse mode is controlled, can bemanufactured.

The above description is given on the method of forming the lowrefractive index portion of the two-dimensional photonic crystal by theoxidized region which is formed through oxidation of a part of theregion of the first semiconductor layer by the oxidizing speciessupplied through the microhole. However, the method of forming the lowrefractive index portion of the two-dimensional photonic crystal is notlimited thereto.

For example, an etching liquid (for example, buffered hydrogen fluoride)with which wet etching can be selectively performed depending on Alcompositional ratio is used such that there can be formed a lowrefractive index portion made of a void through etching in a part of aregion of the first semiconductor layer.

On this occasion, the void may be filled with a material having a lowerrefractive index compared with the first semiconductor layer.

Further, in this embodiment, the current confinement structure is formedin the upper multilayer reflection mirror, but may be formed in thelower multilayer reflection mirror.

Further, in this embodiment, the current confinement structure is formedby selective oxidation, but may be formed by proton implantation. Insuch a case, it is not always necessary to form the mesa structure, andthus surface-emitting laser devices can be easily integrated into asmall area to serve as an array.

Further, in this embodiment, the microhole is filled with the materialhaving a lower refractive index than the first semiconductor layer.Accordingly, compared with the case where the microhole is formed ofair, its mechanical strength is enhanced, and the effects on thesemiconductor multilayer reflection mirror from a sidewall interface,such as oxidation, can be reduced.

Embodiment 2

A surface-emitting laser according to Embodiment 2 of the presentinvention is described.

FIG. 2 is a schematic explanatory view illustrating the surface-emittinglaser according to this embodiment.

In FIG. 2, a surface-emitting laser 200 includes a substrate 205, aresonator 207, a lower multilayer reflection mirror 210, a lower spacerlayer 220, and an active layer 230.

The surface-emitting laser 200 includes an upper spacer layer 240, anupper multilayer reflection mirror 250, and a current confinement layer260.

The surface-emitting laser 200 includes a two-dimensional photoniccrystal structure 270 and a low refractive index portion 275.

The surface-emitting laser 200 includes a microhole 280, an upperelectrode 290, and a lower electrode 295.

In Embodiment 2, the surface-emitting laser 200 has a structure in whichthe resonator 207 is formed on the substrate 205.

In the resonator 207, the lower multilayer reflection mirror 210, thelower spacer layer 220, the active layer 230, the upper spacer layer240, and the upper semiconductor multilayer reflection mirror 250 areformed in the stated order.

In the upper semiconductor multilayer reflection mirror 250, the currentconfinement layer 260 formed of a conductive region 261 and a highresistance region 262 is formed.

Further, the low refractive index portion 275 is formed in a firstsemiconductor layer forming the upper semiconductor multilayerreflection mirror 250, whereby there is formed the two-dimensionalphotonic crystal structure 270 formed of a high refractive index portionand a low refractive index portion which are arranged in a directionparallel to the substrate.

A second semiconductor layer is laminated on the first semiconductorlayer, and the microhole 280 penetrates from an upper surface of the lowrefractive index portion 275 to a middle layer which is not the uppersurface of the second semiconductor layer, that is, to the middle layerwhich is not the surface of the upper semiconductor multilayerreflection mirror 250.

Then, the cross section of the microhole in a direction parallel to thesubstrate is smaller than the cross section of the low refractive indexportion formed in a region of the first semiconductor layer in thedirection parallel to the substrate.

Further, the ring-shaped upper electrode 290 is provided on the surfaceof the upper semiconductor multilayer reflection mirror 250, and thelower electrode 295 is provided on the substrate 205.

In other words, Embodiment 2 has substantially the same structure ofEmbodiment 1 except that, in the upper semiconductor multilayerreflection mirror, the microhole which is in contact with the lowrefractive index portion of the two-dimensional photonic crystal doesnot penetrate to the surface of the upper semiconductor multilayerreflection mirror.

In Embodiment 2, because the volume of the microhole is smaller thanthat of the microhole of Embodiment 1, there is the merit that opticalloss of the upper semiconductor multilayer reflection mirror can befurther suppressed.

Further, because the etching depth required in forming the microhole issmall, there is the merit that the process is more simplified.

Next, a method of manufacturing the surface-emitting laser 200 accordingto Embodiment 2 of the present invention is described. As illustrated inFIG. 2, in this surface-emitting laser device, the lower multilayerreflection mirror 210, the lower spacer layer 220, the active layer 230,the upper spacer layer 240, and the upper semiconductor multilayerreflection mirror 250 are laminated in the stated order on the n-typesemiconductor substrate 205 such as an n-GaAs substrate.

The upper semiconductor multilayer reflection mirror 250 is formed of afirst upper multilayer reflection mirror 251 and a second uppermultilayer reflection mirror 252.

The manufacturing method until the upper spacer layer 240 ismanufactured is performed in the same process as in Embodiment 1.

Next, on the upper spacer layer, the first upper multilayer reflectionmirror 251 formed of 10 pairs of a low refractive index layer and a highrefractive index layer is laminated.

Now, the low refractive index layer in a second pair from the bottom ofthe first upper multilayer reflection mirror 251 is made to serve as thecurrent confinement layer 260 for current confinement, andp-Al0.98Ga0.02As is used therefor, for example. Further, a seventh-pairlow refractive index layer from the bottom is made to serve as the firstsemiconductor layer for forming the two-dimensional photonic crystalstructure 270, and p-Al0.96Ga0.04As is used therefor.

Further, the thickness of the first semiconductor layer is, for example,an optical thickness of 3.25 times an oscillation wavelength.

Next, the second semiconductor layer is laminated on the firstsemiconductor layer for forming the two-dimensional photonic crystalstructure 270, and a resist is applied after the formation of aprotective layer on the laminated film surface, thereby forming apattern similar to Embodiment 1.

Next, using ICP etching method which introduces chlorine gas, the secondsemiconductor layer is provided with multiple microholes 280.

A bottom of the microhole is arranged so as to be located on an uppersurface of the first semiconductor layer which forms the uppersemiconductor multilayer reflection mirror 250 for forming thetwo-dimensional photonic crystal structure 270 as in the case ofEmbodiment 1 described above.

Next, after a resist is removed, water vapor is introduced from themicrohole, and the microhole is subjected to heat treatment at 450° C.,for example.

Accordingly, as in the case of Embodiment 1 described above, atwo-dimensional photonic crystal structure having the low refractiveindex portion 275 provided with a cross section in a direction parallelto the substrate, which is larger than a cross section of the microhole280, is formed in the first semiconductor layer.

The size of the low refractive index portion can be adjusted by changingconditions of the oxidation process, that is, processing time, flow rateof water vapor, and temperature of heat treatment. The size of the lowrefractive index portion is, for example, a size of a circle having adiameter of 1 μm.

Next, in order to manufacture a structure in which the microhole isformed until a middle layer in the course to reach the upper surface ofthe second semiconductor layer, a third semiconductor layer which ismade through regrowth of the crystal is formed on the secondsemiconductor layer. Specifically, after the microhole is filled with,for example, SiO₂ or a resin, the protective layer of the surface of thefirst upper multilayer reflection mirror 251 is removed, and regrowth ofthe semiconductor crystal is performed from the surface of the firstupper multilayer reflection mirror 251.

Accordingly, on the second semiconductor layer, the second uppermultilayer reflection mirror 252 formed of the third semiconductor layerthrough crystal regrowth is formed.

In the regrowth, for example, MOCVD method is employed.

In MOCVD, growth parameters are generally controlled to make the lateralgrowth mode larger than the thicknesswise growth mode.

Specifically, in MOCVD of AlGaAs, V/III is increased (to 500), growthpressure is reduced (to 100 mmHg), and growth temperature is raised (to750° C.). As a result, it is readily possible to secure a diffusionlength equal to or greater than 50 nm.

After the microhole is flattened, it is desirable that the growthconditions be reset to the normal growth mode.

The second multilayer reflection mirror 252 is, for example, amultilayer film formed of 30 pairs of films.

The regrowth can be performed more easily as the area of the hole of alow refractive index medium formed on the surface of the semiconductorbecomes smaller.

Thus, even though the regrowth is difficult to be performed from thetwo-dimensional photonic crystal structure due to the size of the lowrefractive index hole, the regrowth is performed easily from the surfacewhere a microhole smaller than a low refractive index hole of thetwo-dimensional photonic crystal structure is made. After that, as inthe case of Embodiment 1, a circular mesa and a current confinementlayer are formed, and then the upper electrode 290 and the lowerelectrode 295 are formed, whereby the surface-emitting laser 200 can bemanufactured.

In this manner, according to the structure of this embodiment, becauseof the presence of the layer where the microhole having a smaller holediameter compared with the hole of the two-dimensional photonic crystalstructure is made on the hole of the two-dimensional photonic crystalstructure, the crystal regrowth of the semiconductor can be performedthereon.

In other words, the hole diameter of the hole of the two-dimensionalphotonic crystal for controlling a transverse mode is generally equal toor more than several hundreds nm, so it is difficult to directly performa high-quality crystal regrowth of a semiconductor thereon. However,according to the structure of this embodiment, higher-quality regrowthcan be performed.

Accordingly, a high-quality upper semiconductor multilayer reflectionmirror can be laminated, and thus a problem of current injection and aproblem of the process can be solved while controlling the transversemode of the surface-emitting laser.

Embodiment 3

A surface-emitting laser according to Embodiment 3 of the presentinvention is described.

FIG. 3 is a schematic explanatory view illustrating the surface-emittinglaser according to this embodiment.

In FIG. 3, a surface-emitting laser 300 includes a substrate 305, aresonator 307, a lower multilayer reflection mirror 310, a lower spacerlayer 320, and an active layer 330.

The surface-emitting laser 300 includes an upper spacer layer 340, anupper multilayer reflection mirror 350, and a current confinement layer360.

The surface-emitting laser 300 includes a two-dimensional photoniccrystal structure 370 and a low refractive index portion 375.

The surface-emitting laser 300 includes a microhole 380, an upperelectrode 390, and reference a lower electrode 395.

In Embodiment 2, the two-dimensional photonic crystal structure and themicrohole are formed in the upper semiconductor multilayer reflectionmirror, but can be formed in the lower multilayer reflection mirror, asdescribed in this embodiment.

In the surface-emitting laser 300 according to this embodiment, thelower multilayer reflection mirror 310, the lower spacer layer 320, theactive layer 330, the upper spacer layer 340, and the upper multilayerreflection mirror 350 are laminated on the substrate 305.

Besides, the two-dimensional photonic crystal structure 370 and themicrohole 380 are formed in the lower multilayer reflection mirror 310.Such an embodiment is also applicable to the present invention.

In this structure, 35 pairs of the lower multilayer reflection mirrors311 are laminated on the substrate 305, and then the microhole and thephotonic crystal structure are formed.

Then, using crystal regrowth of a semiconductor, a lower multilayerreflection mirror 312, the lower spacer layer 320, the active layer 330,the upper spacer layer 340, and the upper multilayer reflection mirror350 are laminated from a surface of the lower multilayer reflectionmirror 311.

In this embodiment, the lower spacer layer and the active layer areformed after the formation of the photonic crystal structure.

Consequently, the semiconductor multilayer reflection mirror 312 formedon the active layer side is not affected by manufacturing deviation suchas overetching, which stabilizes its characteristics.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

In particular, an appropriate change can be made to a material or ashape of the semiconductor layer, the photonic crystal structure, andthe electrode, a pattern of the refractive index distribution of thephotonic crystal structure, and the like.

For instance, a description has been made on the case where an AlGaAslayer is used as the semiconductor layer in the above embodiments, butan AlAs layer may also be used.

This application claims the benefit of Japanese Patent Application No.2007-198485, filed Jul. 31, 2007, which is hereby incorporated byreference herein in its entirety.

1. A surface-emitting laser, comprising: a plurality of semiconductorlayers laminated on a substrate, the plurality of semiconductor layersincluding a lower semiconductor multilayer reflection mirror, an activelayer, and an upper semiconductor multilayer reflection mirror; and acurrent confinement layer including an oxidized region and anon-oxidized region, wherein the non-oxidized region is formed of one ofAlGaAs and AlAs, wherein a first semiconductor layer includes AlGaAshaving a lower Al compositional ratio than an Al compositional ratio ofthe non-oxidized region, wherein one of the lower semiconductormultilayer reflection minor and the upper semiconductor multilayerreflection mirror includes a first semiconductor layer having atwo-dimensional photonic crystal structure of that includes a highrefractive index portion and a low refractive index portion, which arearranged in a direction parallel to the substrate, and wherein a secondsemiconductor layer laminated on the first semiconductor layer includesa microhole, which reaches the low refractive index portion, a crosssection of the microhole in the direction parallel to the substratebeing smaller than a cross section of the low refractive index portionof the first semiconductor layer.
 2. A surface-emitting laser accordingto claim 1, wherein the microhole is formed to extend from an uppersurface of the first semiconductor layer including the low refractiveindex portion to a position within the second semiconductor layerwithout extending to an upper surface of the second semiconductor layer.3. A surface-emitting laser according to claim 2, wherein a structure inwhich the microhole is formed is constructed by forming a thirdsemiconductor layer through crystal regrowth on the second semiconductorlayer.
 4. A surface-emitting laser according to claim 1, wherein thefirst semiconductor layer has an Al compositional ratio that is higherthan an Al compositional ratio of the second semiconductor layer andhigher than a Al compositional ratio of a semiconductor layer formedunder the first semiconductor layer.
 5. A surface-emitting laseraccording to claim 4, wherein the low refractive index portion includesan oxidized region formed through oxidation of a part of a region of thefirst semiconductor layer by an oxidizing species supplied through themicrohole.
 6. A surface-emitting laser according to claim 1, wherein thelow refractive index portion includes a void formed in a part of aregion of the first semiconductor layer.
 7. A surface-emitting laseraccording to claim 6, wherein the void is filled with a material havinga lower refractive index than that of the first semiconductor layer. 8.A surface-emitting laser according to claim 1, wherein the microhole isfilled with a material having a lower refractive index than that of thefirst semiconductor layer.